! PROJECT SUMMARY Developmental programs are driven by the transcription factors that convert signaling information into the precise patterns of gene expression needed for accurate cell fate transitions. Transcription factor-DNA and transcription factor-transcription factor interactions together produce the necessary regulatory dynamics and precision. Because of the greater complexity and heterogeneity of protein-protein interactions, the field has focused on manipulating DNA sequence to study the mechanisms that determine enhancer specificity and gene expression output. This proposal describes our plan to manipulate transcription factor protein-protein interaction affinity. The experimental plan takes advantage of the deep mechanistic understanding and well- validated in vitro and in vivo assays established over two decades of studying the function and regulation of the conserved ETS family repressor Yan in the developing Drosophila visual system. Aim 1 outlines a multi- disciplinary strategy to engineer a set of yan alleles in which self-association affinity is gradually increased from weak to strong. Using these alleles, Aim 2 tests competing hypotheses for how protein-protein interaction affinity influences Yan function and target gene specificity during photoreceptor specification in the fly retina. Because the ideas we are testing address fundamental mechanisms of transcriptional and developmental regulation, the discoveries that emerge are likely to have impact well beyond our specific model system.